The present invention relates to a spread spectrum communication receiver and, more particularly, to a synchronizer circuit based on a correlation demodulation scheme.
FIG. 7 shows a spread spectrum communication receiver based on a conventional correlation demodulation scheme. Referring to FIG. 7, a received signal is input to a detection correlator 11 and a plurality of peak detection correlators 20. The detection correlator 11 outputs a correlation value relative to a spread code in the chip phase designated by an up-down counter 80. A detector 12 detects the output signal from the detection correlator 11. A decoder 13 decodes the output signal from the detector 12.
The peak detection correlator 20 calculates a correlation value at each sampling point within one period of the spread code. The output from the peak detection correlator 20 is input to an integration circuit constituted by a plurality of adders 30 and a plurality of memories 40. The output from the integration circuit is input to a peak position detector 50. The detected peak position is input, through a switch 60, to a peak position memory 70 in the initial mode, and to a peak comparator 71 in the steady mode.
In this case, the peak comparator 71 performs control to minimize the difference between the peak position within one period of the spread code and the value stored in the peak position memory 70. More specifically, the peak comparator 71 outputs "+1" when the peak position within one period of the spread code is larger than the value in the peak position memory 70; outputs "-1" when the peak value is smaller than the stored value. The up-down counter 80 sequentially adds the outputs from the peak comparator 71. The value in the up-down counter 80 is output to the detection correlator 11. The chip phase of the detection correlator 11 is controlled in accordance with this value, thereby holding synchronization.
A synchronization device for a spread spectrum communication receiver using such a correlation demodulation scheme is disclosed in Japanese Patent Laid-Open No. 7-50613.
In a conventional spread spectrum communication receiver having such an arrangement, when a path for a received signal is to be detected, the reception levels in the entire search range are integrated over a plurality of periods, thereby facilitating detection of a path.
According to this method, however, when a plurality of paths in a multipath fading state in which the reception levels are low and greatly vary are to be detected, detection errors cannot be satisfactorily suppressed. For this reason, erroneously detected paths are also demodulated.
Assume that the correlation level of a received signal is as low as a noise level. In this case, if an instantaneous peak is set as a chip phase for correlation demodulation, a correlation peak cannot be detected owing to the influences of noise and fading. For this reason, peaks based on noise without any correlation component are frequently detected, resulting in a deterioration in demodulation characteristics.